Tracker.
The tracker is typically a third-party that is included by first-party publishers to provide advertising and tracking services on their sites.
We assume that the tracker is able to link the user's different page visits by using well-known cross-site tracking techniques such as cookies or browser fingerprinting.
We consider a strong threat model by assuming that the tracker has complete coverage of a user's browsing profile.
While a tracker typically does not have complete coverage, prior literature has shown that some trackers indeed have significant coverage of top sites and that even trackers with smaller individual coverage collaborate with each other to improve their coverage [1]}, [2]}, [3]}, [4]}.
The tracker is also assumed to have substantial computational resources to train machine learning models on the user's browsing profile to effectively profile the user's interests and target relevant ads [5]}.Note that while tracking may take place via additional modalities like location, browsing profile based profiling and targeting remains one of the most important modalities currently used in the online advertising ecosystem. See Section REF  for more discussion.
